Liberty Street Cyclery's (171 East Liberty #125, 416-516-8880, libertycyclery.com) Edmund Tsui on servicing a bike stashed indoors for the winter versus one ridden through the snowy season:
Assuming that a bike was put away in good mechanical condition, there shouldn't be any major issues in the spring. Air in the tires and lube on the chain should be all it needs to get going.
Riding through the winter destroys bikes. All the salt, road grit and water act like a wet sander in the drivetrain, grinding away at the chain ring and cassette teeth and wearing out the chain. All the seals on the bike have to work extra-hard to protect the bearings, and the brake pads wear down the rims faster. Most bikes that have seen winter duty typically need a tune-up to check cables, brake pads and the drivetrain come spring.
Bikesauce's (235 Broadview, bikesauce.org) Anibal Davila on tuning up the bike you forgot outside all winter (doh!):
Your chain is probably rusted to hell. Oil it, hang your bike up and watch the chain as you turn the pedals. If you can see stiff links that are not flexing smoothly around the cogs, you either need to get them moving or replace the chain.
If your brakes are seized or sluggish, replace them both. Don't just oil them and go. There could be a lot of rust inside the housings, leading to a weakened cable that will snap when you need it most.
And if your shifters aren't moving, chances are you'll have to replace your cables and housing. You can oil your derailleurs and try to get them working, but honestly, you'll probably end up at a bike shop or a DIY to get them readjusted.
Bateman's Bicycle Company's (29A Barton, 416-538-2453, batemansbicyclecompany.com) Rob Bateman on the essentials for a home bike tool kit:
First, a floor pump with gauge. I see people ridding around on bike tires that are not inflated to proper air pressure, and the result is more work, more flat tires and frequent accidents. Second, a multi-tool with Allen keys will allow you to safely tighten more working parts on a bike. Third, bike de-greaser and chain lube are key to making the bike last. Finally, for the do-it-yourself type, YouTube is a great source for how-tos on basic bike repair.